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Four enzymes, namely, the maize C4-specific phosphoenolpyruvate carboxylase (PEPC), the maize C4-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C3-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO2 assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO2 assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO2 assimilation, however, was more marked at higher CO2 concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO2 inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C4-like photosynthetic pathway into C3 plants, are discussed.

Four enzymes, namely, the maize C[sub]4-specific phosphoenolpyruvate carboxylase (PEPC), the maize C[sub]4-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C[sub]3-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO[sub]2 assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO[sub]2 assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO[sub]2 assimilation, however, was more marked at higher CO[sub]2 concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO[sub]2 inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C[sub]4-like photosynthetic pathway into C[sub]3 plants, are discussed.

Four enzymes, namely, the maize C sub(4)-specific phosphoenolpyruvate carboxylase (PEPC), the maize C sub(4)-specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C sub(3)-specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO sub(2) assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO sub(2) assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO sub(2) assimilation, however, was more marked at higher CO sub(2) concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO sub(2) inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C sub(4)-like photosynthetic pathway into C sub(3) plants, are discussed.

Four enzymes, namely, the maize C 4 -specific phosphoenolpyruvate carboxylase (PEPC), the maize C 4 -specific pyruvate, orthophosphate dikinase (PPDK), the sorghum NADP-malate dehydrogenase (MDH), and the rice C 3 -specific NADP-malic enzyme (ME), were overproduced in the mesophyll cells of rice plants independently or in combination. Overproduction individually of PPDK, MDH or ME did not affect the rate of photosynthetic CO 2 assimilation, while in the case of PEPC it was slightly reduced. The reduction in CO 2 assimilation in PEPC overproduction lines remained unaffected by overproduction of PPDK, ME or a combination of both, however it was significantly restored by the combined overproduction of PPDK, ME, and MDH to reach levels comparable to or slightly higher than that of non-transgenic rice. The extent of the restoration of CO 2 assimilation, however, was more marked at higher CO 2 concentrations, an indication that overproduction of the four enzymes in combination did not act to concentrate CO 2 inside the chloroplast. Transgenic rice plants overproducing the four enzymes showed slight stunting. Comparison of transformants overproducing different combinations of enzymes indicated that overproduction of PEPC together with ME was responsible for stunting, and that overproduction of MDH had some mitigating effects. Possible mechanisms underlying these phenotypic effects, as well as possibilities and limitations of introducing the C 4 -like photosynthetic pathway into C 3 plants, are discussed.